1. Field of the Invention
The present invention relates to a shaft that is installed in an automobile and in various types of industrial machine and a manufacturing method thereof.
2. Description of the Related Art
Conventionally, a shaft used in a constant velocity joint, or the like, has a shank having a splined portion provided at an outer peripheral surface. Furthermore, a large diameter portion having a diameter larger than a diameter of the outer periphery of the splined portion is provided at an end portion of the splined portion in the direction that a fitting member such as an inner race is inserted onto the splined portion. When the fitting member is fitted, the large diameter portion acts as a stopper for stopping the fitting member.
In order to make this type of shank, first, as shown in FIG. 9 (a), a shank 100 having a main portion 105 with a constant radius with respect to a shaft axis; a slanted portion 102; and a large diameter portion 101 is prepared. The slanted portion 102, as shown in FIG. 9 (b), is formed from a tapered portion 102a and a curved portion 102b that extend continuously from a portion adjacent to the large diameter portion 101.
When this shank 100 is machine formed, as shown in FIG. 10 (a), a small diameter portion 103 and a splined portion 104 are formed in the main portion 105 in this order, from the larger diameter 101 portion side thereof. The small diameter portion 103 is provided so as to inhibit interference between the large diameter portion 101 and cutting tool when the splined portion 104 is being machined. In addition, the diameter of the small diameter portion 103 is constant along its entire length. Further, normally, the diameter of the small diameter portion 103 is larger than a diameter of a groove portion 104a of the splined portion 104, and smaller than a diameter of the main portion 105 prior to machining of the splined portion 104, A cut back portion 104b is formed at a portion that runs from the groove portion 104a of the splined portion 104 to the small diameter portion 103. This cut back portion 104b, as shown in FIG. 10 (b), is formed from a tapered portion 104b2 and a curved portion 104b1 that extend successively from the small diameter portion 103 side.
As shown in FIG. 11, a fitting member 200, which acts as an inward joint member of the constant velocity joint, having a splined portion 204 that faces the large diameter portion 101 is fitted onto the shank 100 configured as described above. At this time, the fitting member 200 abuts with the slanted portion 102. When this occurs, load is applied from the fitting member 200 to the slanted portion 102 (in the direction shown by the arrow in FIG. 10(b)). Moreover, in the constant velocity joint, load applied to a tooth flank of the splined portion 104 is applied to the cut back portion 104b and stress concentration occurs between the cut back portion 104b and the small diameter portion 103.
In order to more fully understand the state of this stress concentration with the above described conventional shaft, as an example, FIG. 7 shows tensile stress exerted in the direction along with the shaft axis of the shank 100 when the fitting member 200 is fitted. It should be noted that, as shown in FIG. 11, a starting position on the side of the large diameter portion 101 of the splined portion 104 of the shank 100 is taken as P1, and a fitting starting position of the splined portion 104 for fitting with the fitting member 200 is taken as P2.
As shown by FIG. 7, it is clear that substantial tensile stress is exerted in an area between P1 and P2, and in the area from P2 to the vicinity of the shank end surface side.
Accordingly, a structure has been proposed in Japanese Patent Publication Laid-Open No. Hei. 09-042303 paragraph [0015] and [0016], and FIG. 2 for reducing this stress concentration of the small diameter portion 103 between the cut back portion 104b and the large diameter portion 101.
This structure is configured such that a smooth portion with a diameter smaller than the groove portion 104a of the splined portion 104 is provided at the small diameter portion 103.
However, although stress concentration in the vicinity of the cut back portion 104b is reduced in the case of the invention disclosed in Patent JP 09-042303, the portion with the small diameter is formed in the shank itself. In other words, the smooth portion with the diameter smaller than the diameter of the groove portion 104a of the splined portion 104 is formed in the shank, and thus the strength of the shank is impaired.